1. Field of the Invention
The present invention relates to a golf club head and more particularly to a wood or iron golf club head which is made of metal.
2. Prior Art
In one type of conventional golf club head, the ball-striking member and shaft insertion member are formed as an integral unit by casting or forging from a metal material having the same composition. In another type of conventional golf club head, the ball-striking member and shaft insertion member are formed separately from a metal materials having the same composition or from different metal material having different specific gravities, so that the ball-striking member and shaft insertion member are joined together into an integral unit.
In these conventional golf club heads, however, the material density of the ball-striking member and the material density of the shaft insertion member are selected so as to be the same; accordingly, when an improvement is desired in ball-striking characteristics such as shock resistance, etc., it is necessary to change the cross-sectional shape of a ball-striking part of the ball-striking member or differentiate the compositions of the ball-striking member and shaft insertion member.
Recently, a shaft made from, for instance, a carbon-fiber-reinforced plastic (CFRP), which is a different material from the shaft insertion member made of metal, has been generally used; and when this type of shaft is inserted into a hole formed in the shaft insertion member so as to be bonded thereto, it is not always possible to obtain a strong bonding force from the adhesive agents used. This is because no adhesive agent is known which has a sufficient affinity for both the shaft and the shaft insertion member, which are made from different materials as described above.
Furthermore, the shaft which is inserted into the shaft insertion hole of the shaft insertion member generally has a tapered shape which gradually increases in diameter from the tip end portion to the grip end portion. Accordingly, in order to secure a good bonding strength, it is necessary to form the internal shape of the shaft insertion hole so as to perfectly coincide with the taper-form external shape of the tip end portion of the shaft so that a surface contact is made between the inner circumferential surface of the shaft insertion hole and the outer circumferential surface of the shaft.
However, forming such a shaft insertion hole requires a high degree of precision that is based on skill. If the hole is formed imperfect, the inner circumferential surface of the shaft insertion hole and the outer circumferential surface of the shaft make a point contact (and not a surface contact) with each other. As a result, the bonding surface area is conspicuously small, and the bonding strength is not sufficiently high. Consequently, the bonded area between the two parts are destroyed by the impact that occurs when the ball is hit during long-term repeated use, and the shaft eventually falls out of the shaft insertion hole.
Another means to increase the bonding strength between the shaft and shaft insertion hole is to roughen the outer circumferential surface of the shaft and the inner circumferential surface of the shaft insertion hole. With the roughened surfaces, the anchoring effect of the adhesive agent can increase. However, roughening of the inner circumferential surface of the shaft insertion hole is extremely difficult, though the outer circumferential surface of the shaft can, comparatively, be easily roughened.